This project aims to develop new approaches to evaluation of human vestibular function, with the ultimate aim of improving pathophysiologic understanding, diagnosis, and management of the large number of patients with dizziness and disequilibrium. A central theme is to relate quantitative test performance to lesions of specific vestibular sensory organs or brain regions, through application of selective stimuli that by their transient nature can isolate vestibular responses from non-labyrinthine compensatory mechanisms. We propose to develop very high time resolution tests of vestibular function in response to angular and linear motion, and correlate the finding with other quantitative physiologic and pathologic indicators of vestibular structure and function in patients with clinically well-characterized vestibular disorders. We will employ magnetic search coil sensors to make precise measurements of eye and head movements to investigate in young and older human subjects the effects of aging, well-characterized forms of vestibulopathy, and cerebellar degeneration on vestibulo-ocular (VORs). We will employ precise angular and linear stimulation directed toward specific vestibular sensory organs to characterize their functions and central integration. Specific aims are: to evaluate transient pitch and roll VOR in the planes of individual semicircular canals during transient passive, whole-body rotation under visually salient conditions, and its interaction with otolith stimulation, in normal subjects and in patients with well-characterized surgical lesions, specific vestibular syndromes, and specific cerebellar degenerations; 2) to quantify the heave and surge transient linear c and its kinematic dependence on three-dimensional target location during whole-body translation, and the effects of well-characterized surgical lesions, specific vestibular syndromes, and specific cerebellar degenerations defined by molecular pathology; and, 3) to characterize the chronology of ocular motor adaptations to acute vestibular deafferentation in humans, including recovery of the yaw, pitch, and roll angular VORs, and heave and surge linear VORs, as well as development of extralabyrinthine compensatory mechanisms. Development of these novel tests will direct diagnostic inferences and should facilitate understanding of pathophysiology and the functional efficacy of compensatory mechanisms in patients with vestibular disease.